An electronic device needs a stable power supply; therefore, the power supply device thereof has to provide a stable power supply for the electronic device. Generally, a power supply device is coupled to a power source to acquire an input power, and the power conversion unit of the power supply device converts the input power into a driving power and sends the driving power to loads. However, the conventional design has only one power source. The power source may be a commercial power or another power supply device. When the commercial power or the source power supply device fails, the electronic device will shut down. When the input power drops, the booster unit of the power supply device will be overburdened and thus damaged. Traditionally, a UPS (Uninterrupted Power System) is used to prevent the problems caused by the failure of a single power source. The UPS connects with the power supply device and provides a stable driving power for the electronic device when the power source fails, whereby the problem of abrupt power interruption is avoided. However, the UPS and the power supply device use the same power source. Therefore, the UPS cannot actively switch between different power sources but can only passively prolong the operation of the electronic device for a while. A Taiwan patent No. 1269518 disclosed a “Power Supply System Having AC Redundant and Wrong Plugging Prevention Functions”, and a Taiwan patent No. 561676 disclosed a “Power Supply Device Having an AC Redundant Function” having the same circuit architecture as the former patent, wherein three DC power supply modules are connected in parallel, and two of them are respectively coupled to two different AC power sources, and a switching circuit switches the third one to connect with one of the two AC power sources. When one of the two AC power sources fails, the switching circuit can keep two DC power supply modules operating. However, the prior arts need three power supply modules and have the disadvantages of bulkiness and high price, which limit their applications. A Taiwan patent No. 507974 disclosed a “Power Supply Device Having Dual AC Power Sources Connected in Parallel”, which detects whether the voltage input from a first power source is sufficient to excite the coil of a relay and then determines whether to turn on the input of the first power source. In fact, the prior art cannot really discriminate the levels of the two power sources.